In recent years, due to the energy supply problem, the use of heavy oils has become more common in the fuel burner industry. In the past, oil preheaters have pretty much been controlled by bimetal operated switches or thermocouples. In the case of thermocouples, the associated control equipment could be remotely located, but was relatively expensive. In the case of bimetal sensed oil preheaters, the bimetal had to be mounted, along with its control circuit at the oil preheater.